1. Field of the Invention
The present invention relates to a method and device for measuring a surface potential distribution, a method and device for measuring an insulation resistance, an electrostatic latent image measurement device, and a charging device.
2. Description of the Related Art
FIG. 12 shows the composition of an electrophotographic image forming apparatus in which an electrostatic latent image is formed on the photoconductor drum by the imaging process. To output an image by the electrophotographic image forming apparatus, including the copiers and the laser printers, the following imaging process is usually carried out.
As shown in FIG. 12, the image forming apparatus comprises the charging unit 101, the exposure unit 102, the development unit 103, the transfer unit 104, the fixing unit 105, the cleaning unit 106, and the electric erasing unit 107 which are arranged around the photoconductor drum 108. And the imaging process which includes the following steps (1) to (7) is repeated for every image formation:
(1) Charging: the photoconductor is charged uniformly.
(2) Exposure: light is irradiated to the photoconductor, the electric charge is removed partially, and the electrostatic latent image is formed on the photoconductor.
(3) Development: the electrostatic latent image is converted into the visible image by using the charged particles (toner).
(4) Transferring: the visible image developed is transferred to paper or other recording material.
(5) Fixing: the transferred image is heated and is fixed to the recording material.
(6) Cleaning: the remaining toner on the photoconductor is cleaned.
(7) Electric Erasing: the residual charge on the photoconductor is eliminated.
Each process factor and process quality in each of the above steps of the imaging process affect the final output image quality greatly. In recent years, in addition to high image quality, the demands for increasing the durability, the stabilization, and the energy saving, are made, and it is necessary to raise the process quality of each of the above imaging process steps.
In the imaging process, the electrostatic latent image formed on the photoconductor by the exposure is the factor which directly affects the action of the toner particles, and it is necessary to measure accurately the surface potential distribution on the photoconductor surface. For this reason, it is important to evaluate the quality of the electrostatic latent image on the photoconductor. The process quality of each of the imaging process steps can be improved by measuring the electrostatic latent image of the photoconductor and feeding back to the design.
The method of observing or measuring a surface potential distribution on the surface of an object by the scanning of electron beam is known (for example, Japanese Laid-Open Patent Application No. 03-049143).
In this method, the electron beam is scanned on the measurement sample having a surface potential distribution, and the secondary electron generated on the measurement sample associated with the scanning of the electron beam is detected so that the surface potential distribution is observed or measured. The measurement sample used as a candidate for measurement must be, for example, an LSI chip which is capable of retaining electric charge on its surface for a sufficiently long time.
The electrostatic latent image which is formed on the photoconductor drum in the analog or digital electrophotographic copiers or laser printers by the charging and exposure is a factor which directly affects the action of the toner particles, and, for this reason, the quality evaluation of the electrostatic latent image on the photoconductor is important.
If the electrostatic latent image on the photoconductor is measured and the result of measurement can be fed back to the design of the copiers or the printers, then it is possible to attain improvement in process quality in the charging process or the exposure process. Consequently, the further improvement in the quality of image, the durability and stability, or the energy saving can be expected.
However, the electrostatic latent image formed on the photoconductor may disappear for a short time (about dozens seconds) due to the dark decaying of the photoconductor, and there is little time which can be used for the measurement. If the method of Japanese Laid-Open Patent Application No. 03-049143 is used to measure the surface potential distribution, the electrostatic latent image on the photoconductor will disappear in the preparation stage of the measurement, and practically the measurement cannot be performed.
Furthermore, there is proposed the technique of measuring the electrostatic latent image on the photoconductor having the dark decaying (for example, see Japanese Patent Applications No. 2003-295696 and No. 2004-251800 which are assigned to the assignee of the present invention).
The well-known method of measuring the surface potential distribution uses the sensor head, such as the cantilever, which is brought close to the sample having the surface potential distribution, and the electrostatic attraction and induced current produced at that time as the interaction are measured, and the measured current is converted into the potential distribution. For example, the electrostatic-attraction type is marketed as the SPM (Scanning Probe Microscope). And there are some known techniques of the induced current type (for example, Japanese Patent No. 3009179 and Japanese Laid-Open Patent Application No. 11-184188).
However, in order to use the known method, it is necessary to bring the sensor head close to the sample. In order to obtain the 10-micrometer spatial resolution in this case, it is necessary to set the distance between the sensor and the sample to 10 micrometers or less.
When taking into consideration the proximity conditions, there are the following problems. Although the method can be uses for other applications, it cannot practically be used for the measurement of the electrostatic latent image. Namely, the distance measurement is needed absolutely, the measurement takes time and the latent-image state changes during the measurement, the electric discharge and adsorption take place, and the sensor itself disturbs the electric field.
For this reason, the method of visualizing the electrostatic latent image is usually taken as the practical measurement method. That is, the electric charge is given to the toner in the form of colored powder-like particles, the development is performed by the Coulomb force which works between the charged toner and the electrostatic latent image, and the surface potential distribution of the electrostatic latent image is measured by transferring this toner image to the paper or the recording tape.
However, according to the above method, there is the problem that it does not mean the measurement of the surface potential distribution of the electrostatic latent image itself since the measurement is performed after the processes of development and transferring are performed,
As mentioned above, practically the electrostatic latent image cannot be measured by the method of irradiating the electron beam to the sample which is capable of retaining electric charge on its surface (the LSI chip), and the method of bringing the sensor head, such as the cantilever, close to the sample, and measuring the electrostatic attraction and the induced current.
To overcome the above problem, there is proposed the technique of measuring the electrostatic latent-image distribution in which the charged particle beam or electron is irradiated, in advance, to the sample (for example, Japanese Laid-Open Patent Applications No. 2003-295696 and No. 2003-305881 which are assigned to the assignee of the present invention). In addition, the electrostatic latent image means the image in the state where the electric charge is distributed over the dielectric substance.
Namely, in the proposed technique, if the potential distribution occurs on the surface of the sample when the electron is irradiated, the electric field distribution according to the surface potential distribution is formed in space. For this reason, the secondary electron generated by the incident electron is put back by the electric field, and the quantity of the secondary electron which reaches the detector is decreased.
By using the above-described feature, the portion with the high field intensity becomes dark and the portion with the low field intensity becomes bright so that the contrast is formed, and the contrast image according to the surface potential distribution can be detected. Therefore, the exposed portion becomes black and the non-exposed portion becomes white after the exposure, so that the thus formed electrostatic latent image can be measured.
FIG. 13 shows the relation between the measured electric field intensity obtained by detecting the secondary electron generated by the incident electron and the light-and-dark image produced according to the surface potential distribution.
It is possible to measure the electrostatic latent image by using the above method. However, as shown in FIG. 13, even if the electric field intensity is high, the contrast image will not necessarily become the luminance signal proportional to the electric field intensity, and the contrast image is likely to become the light-and-darkness image with the tendency of the binary signal.
In addition, it is assumed that, concerning the electric field intensity described, the sign of the electric field vector on the side of the incidence electron in the direction perpendicular to the sample surface is positive.
As for the contrast image of lightness and darkness, as illustrated, the threshold level is in the vicinity of the level where the electric field intensity is zero. However, there is the electric field intensity (Eth) which is required for attachment of the charged particles (toner) in the actual electrophotographic printing process, and it is important to measure the diameter of the latent image at the level of Eth.
If the electric field intensity distribution can be approximated with a smooth curve like the Gaussian distribution, it is possible to compute the diameter of the latent image at the Eth level. However, the error components may be contained in the actual case, and it does not usually become the smooth curve like the Gaussian distribution. Acquiring directly the image as the contrast image at the Eth level is necessary for the practical case.
Moreover, FIG. 14 shows the image acquisition method (Japanese Laid-Open Patent Application No. 03-049143) in which the back bias voltage is applied in the composition of the sample and the electrode. In this image acquisition method, the voltage of the back bias is applied to the back surface of the sample 32 in order to obtain a proper potential distribution, and the electrostatic latent image according to the applied electron beam is acquired.
However, in this method, the electric field intensity on the front surface of the sample is not affected even when the voltage of the back bias is applied to the back surface of the sample 32. The effect that the threshold level of the potential distribution is changed cannot be produced. In addition, in the composition of FIG. 14, the reference numeral 33 denotes the conductor, 34 denotes the secondary electron detector, 35 denotes the power supply, and 37 denotes the objective lens, respectively.